1. Field of the Invention
The present invention relates to a diaphragm/spring actuated pressure relief valve or back-pressure regulator that is pressure balanced against the outlet pressure for use on tanks where the outlet may be exposed to a pressure or vacuum that can vary in magnitude having improved pressure sensing.
2. Related Art
There are many types of diaphragm/spring actuated pressure relief valves.
The outlet ports of two such commercial valves are unbalanced against outlet pressure, i.e., the opening point of the valves are affected (changed) by the amount of pressure or vacuum on the outlet side due to the valve seat being biased to the outlet pressure. If the diaphragm of either valve ruptures, the spring will keep the valve closed and it will not be able to relieve the full rated capacity of the valve. The most that one of these valves can relieve is the amount that can flow through the small passageway(s) that leads to the diaphragm chamber. In the other valve the maximum relief capacity is whatever can flow through the small threaded port in the spring bonnet, assuming the internal passageways are larger than the spring bonnet port.
A third valve type utilizes a balancing diaphragm to overcome the effects of varying outlet pressure. If the main diaphragm ruptures, the spring will keep the valve closed and it will not be able to relieve the full rated capacity of the main valve. The most that the valve can relieve is the amount that can flow through the relatively small passageways leading to the main diaphragm. Therefore a ruptured main diaphragm can be hazardous and compromise the safety of the system that the valve is intended to protect by not being able to relieve the unwanted pressure build-up.
One of the criteria of determining how well a pressure relief valve performs is the amount of "overpressure" or accumulation required to achieve a certain relieving capacity. "Overpressure" or accumulation is that increment of pressure higher than the set pressure, which is the pressure at which the valve begins to open. Most relief valves that bear the ASME UV stamp, whether they are spring loaded or pilot operated, are rated for capacity at 10% overpressure. Weight loaded tank vents, valves that are closed by a weighted cover over the opening, require a significant overpressure to have meaningful flow for its size. At 10% overpressure weight loaded tank vents do not flow much. Some designs require as much as 50% to over 100% overpressure for significant flow. Prior designs of weight or spring loaded pressure relief valves are not able to open very wide at relatively small overpressures because when the valve opens and relieves, the full tank pressure is not communicated to the sensing chamber or seat due to pressure drops along the sensing path. A smaller more efficient valve can often times be used in place of a larger valve of lower efficiency resulting in savings in cost, weight, and space requirements, with associated benefits.
U.S. Pat. No. 5,845,675 discloses a diaphragm/spring operated pressure relief valve having a sensing passage of substantially the same flow area as the outlet orifice of the valve whereby failure of either diaphragm results in "fail open" configuration. The present diaphragm/spring actuated pressure relief valve uses the same features of U.S. Pat. No. 5,845,675 with the addition of a shrouded sensing inlet located to capture the total pressure at the valve inlet and channel it to the sensing chamber. This arrangement reduces the pressure drop along the sensing path which allows the sensing diaphragm to more accurately sense the tank pressure and lift open the valve seat wider when an increase in tank pressure is sensed, thereby improving the performance of the valve.